This invention relates generally to distillation, and more particularly concerns highly efficient distillation methods and systems employing heat pumps.
The operation of a heat pump involves, for example, the use of work .DELTA..omega. to raise the heat of a fluid from a relatively lower temperature T.sub.c to a relatively higher temperature T.sub.H. See for example the following equation: EQU .DELTA..omega.+Q.sub.c =Q.sub.H ( 1)
where
.DELTA..omega.=work input PA1 Q.sub.c =heat content (of fluid) at T.sub.c PA1 Q.sub.H =heat content (of fluid) at T.sub.H PA1 (a) a heat pump having first and second fluid passing portions, the pump operable to transfer heat to distillable fluid in said first portion from coolant in said second portion and from a heat input source, PA1 (b) distillation means having first and second zones, the first zone connected with the heat pump first portion to receive heated fluid therefrom for condensation at said first zone, the second zone extending in latent heat receiving proximity to said first zone, the second zone connected with the heat pump second portion to circulate heated coolant thereto after transfer of latent heat to said coolant in said second zone.
The coefficient of performance of the heat pump is defined, in warming mode, as: ##EQU1## and in cooling mode, as: ##EQU2##
It will be noted that C.sub.p.omega. and C.sub.pc are each greater than 1, in value. Generally, it is found that commercial heat pumps have coefficients of performance ranging from about 1.5 up to about 5. Note that it is desired to operate a heat pump with as low a .DELTA..omega. value as possible, in order to conserve energy input, and that the lower .DELTA..omega., for a fixed Q.sub.H or Q.sub.c, the higher the coefficient of performance.
In distillation technology, hot feed water is vaporized, the vapor then being condensed. It is found that one pound of water requires about 150 BTU's of energy to raise its temperature from 62.degree. F. (ambient) to 212.degree. F. (boiling temperature at one atmosphere of pressure); further, that one pound of water then requires about 1000 BTU's of energy input to vaporize it. Conversely, when the vapor is condensed, it releases 1,000 BTU's. In the case of one pound of seawater, about 1 to 2 BTU's is theoretically required to remove the salt content.
It is seen from the above that straight-forward distillation to remove salt from seawater requires from 500 to 1,000 times more energy than theoretically is necessary to remove the salt content from one pound of such seawater. Such straight-forward distillation does not contemplate recovery of the latent heat of condensation from the product distillate, so that it becomes imperative to recover that latent heat if efficiency is to be achieved in terms of using only 1 to 2 BTU's energy to remove the salt. In this regard, and for purposes of comparison, other processes to remove salt require the following listed BTU input ranges to remove salt from one pound of seawater:
______________________________________ Process Energy input Req'd. ______________________________________ reverse osmosis 25-50 BTU solar still (multiple effect) 25-100 " flash evaporation still 100-200 " (multiple effect) centrifugal still about 28 " ______________________________________
All of these required energy inputs are far too high in relation to the 1-2 BTU's theoretically required.